1. Field of the Invention
The present invention relates to radiation dosimetry systems and, particularly, to accurate calculation of the equivalent absorbed dose due to a radiation exposure event.
2. Background of the Invention
Occupational radiation exposure events can occur in healthcare, the oil and gas industry, the military and other industrial settings where the use of materials or devices that emit ionizing radiation can result in accidental or occupationally unavoidable exposure events.
Emergency radiation exposure events can occur when a Radiological Dispersal Device (RDD), Improvised Nuclear Device (IND), or another source of radioactive material is released and contaminates a given area.
Radiation dosimetry programs for occupational radiation exposure events, and Emergency management plans for emergency radiation exposure events, have been developed to monitor and protect workers or civilians who might be exposed to radiation during a radiation event.
An important aspect of any emergency management plan following a radiation event is to ensure the safety of fire, police and other emergency response personnel (“First Responders”), health-care workers, and citizens that might be exposed to radiation resulting from the radiological or nuclear device. Radiation exposure of first responders and healthcare workers is often, at least partially, monitored using traditional radiation detecting devices; however, monitoring the exposure of potentially tens of thousands of civilians presents a more difficult problem.
Furthermore, after the removable contamination has been eliminated, there may be a need for ongoing, external personal dosimetry monitoring for individual First Responders, healthcare workers, and members of the public. Site restoration could be a lengthy project and, to minimize disruption to society, it may be necessary to allow inhabitants to have access to certain areas before cleanup is complete. For example, allowing citizens to pass through transit centers, thoroughfares, or certain areas of buildings would facilitate government operations, commerce, uniting of families, routine medical treatments, etc. As an individual moves through a contaminated area, it would be valuable to know the dose and time of exposure at each location visited. Such dose measurements could reduce reliance on model-based estimates of dose, and avoid unnecessary area restrictions by providing a geographic map of the dynamic dose distribution reconstructed from a large number of dosimeters collecting dose event data over the potentially still-contaminated area. Unlike cleanup at decommissioned facilities where the public could be excluded with little cost to society, in an urban environment, time is of the essence and the cost of exclusion may be greater than the benefit avoiding exposure to a relatively low radiation dose. After cleanup, personal dosimetry could boost public confidence that their personal dose is below acceptable thresholds, and that the final cleanup was effective.
Several radiation measurement technologies currently exist including TLD dosimeters, OSL dosimeters, electronic dosimeters, quartz or carbon fiber electrets, and other solid-state radiation measurement devices.
Thermoluminescent Dosimeter (TLD) badges are personal monitoring devices using a special material (e.g., lithium fluoride) that retains deposited energy from radiation. TLD badges are read using heat, which causes the TLD material to emit light that is detected by a TLD reader (calibrated to provide a proportional electric current). Significant disadvantages of TLD badges are that the signal of the device is erased or zeroed out during reading, the dosimeters must be returned to a processing laboratory for reading, and substantial time is required to obtain the reading.
Optically Stimulated Luminescence (OSL) badges use an optically stimulated luminescent material (OSLM) (e.g., aluminum oxide) to retain radiation energy. Tiny crystal traps within the OSL material trap and store energy from radiation exposure. The amount of exposure is determined by illuminating the crystal traps with a stimulating light of one color (e.g., green) and measuring the amount of emitted light of another color (e.g., blue). Alternatively, pulsed light stimulation can be used to differentiate between the stimulation and emission light, as describe for example in U.S. Pat. Nos. 5,892,234 and 5,962,857, which are incorporated herein by reference in their entireties. Unlike TLD systems, OSL systems provide a readout in only a few seconds and, because only a very small fraction of the exposure signal is depleted during readout, the dosimeters can be readout multiple times. OSL dosimeters can be read in the field using small, field-transportable readers, however, the readers are still too large, slow and expensive to allow individual, real-time readings in the field. In currently-existing OSL dosimetry programs for reporting the dose of record, the dosimeters must be returned to a processing laboratory for readout.
For more information on OSL materials and systems, see, U.S. Pat. No. 5,731,590 to Miller; U.S. Pat. No. 6,846,434 to Akselrod; U.S. Pat. No. 6,198,108 to Schwietzer et al.; U.S. Pat. No. 6,127,685 to Yoder et al.; U.S. patent application Ser. No. 10/768,094 to Akselrod et al.; all of which are hereby incorporated by reference in their entireties. See also, Lars Botter-Jensen et al., Optically Stimulated Luminescence Dosimetry, Elesevier, 2003; Klemic, G., Bailey, P., Miller, K., Monetti, M., “External radiation dosimetry in the aftermath of radiological terrorist event,” Rad. Prot. Dosim, in press; Akslerod, M. S., Kortov, V. S., and Gorelova, E. A., “Preparation and properties of Al2O3:C,” Radiat. Prot. Dosim. 47, 159-164 (1993); and Akselrod, M. S., Lucas, A. C., Polf, J. C., McKeever. S. W. S., “Optically stimulated luminescence of Al2O3:C,” Radiation Measurements, 29, (3-4), 391-399 (1998), all of which are incorporated by reference in their entireties.
Solid State Sensors use solid-phase materials such as semiconductors to quantify radiation interaction through the collection of charge in the semiconductor media. As the radiation particle travels through the semiconductor media electron-hole pairs are generated along the particle path. The motion of the electron-hole pair in an applied electric field generates the basic electrical signal from the detector. There are two main categories of solid state sensors, active and passive. Active sensors often use a semiconductor that is biased by an externally powered electric field that requires constant power. The active sensors generate electric pulses for each radioactive particle striking the sensor. These pulses must be continuously counted to record the correct radiation dose. A loss of power means no dose is measured. Active solid state sensors are typically made from silicon and other semiconductors. Passive solid state sensors utilize an on-device charged medium that maintains the electric field necessary to separate the electron-hole pairs without drawing external power. Passive solid state dosimeters often use what is called a floating gate where the gate is embedded within the detection medium so it is electronically isolated. The floating gate is charged and provides the electric field for charge separation, for example, see U.S. Pat. No. 6,172,368 to Tarr. The medium above the floating gate is typically an insulator such as silicon oxide however it can also be a sealed gas chamber, for example see U.S. Pat. No. 5,739,541 to Kahilainen. Passive Solid state electronic detectors offer a means of monitoring radiation that are compatible with the present invention.
Electronic dosimeters are battery powered, and typically incorporate a digital display or other visual, audio or vibration alarming capability. These instruments often provide real-time dose rate information to the wearer. For routine occupational radiation settings in the U.S. electronic dosimeters are mostly, but not strictly, used for access control and not for dose of record. A number of cities and states issue electronic dosimeters to HAZMAT teams as part of their emergency response plans. There are presently tens of thousands of electronic dosimeters deployed, for example, for homeland security purposes; however, electronic dosimeters are impractical for widespread use dosimeters due to their high cost.
Quartz or carbon fiber electrets are cylindrical electroscopes where the dose is read by holding it up to the light and viewing the location of the fiber on a scale through an eyepiece at one end, and a manually powered charger is required to zero the dosimeter. The quartz fiber electret is an important element of many state emergency plans. For example, some plans call for emergency responders to be issued a quartz fiber electret along with a card for recording the reading every 30 minutes, as well as a cumulative dosimetry badge or wallet card. While they are specified for use in nuclear power plant emergencies, the NRC does not require that they be NVLAP accredited but, rather, only that they be calibrated periodically.